The present invention relates to a memory backup circuit and a memory backup method and, more particularly, to a memory backup circuit and a memory backup method suitable for dynamic RAM (called "DRAM", hereinafter) used as an image memory in a facsimile apparatus and the like.
Generally DRAM is used as an image memory of a facsimile apparatus. Because of its construction, the DRAM has to be recursively rewritten with information to be stored in a predetermined time period, and this process is called "refresh", otherwise the contents in the DRAM will be lost.
The main purpose to use this conventional memory in a facsimile apparatus is to store an image to be send later or an image received while shortage of a printing paper.
As a printing unit of the facsimile is changed to a laser beam printer and an ink-jet printer which print images on paper sheets, a time gap between a receiving operation and a printing operation has arisen. Accordingly, the memory becomes often used as a temporary storage memory for storing image data for short time period since the image data is received until it is printed out.
As the purpose to use the DRAM has been changed as described above, DRAM having large capacity and consuming less electricity has been developed as well as it has become possible to backup with a relatively simple control (generally called as "self-refresh"), it has become possible to backup DRAM for a couple to several hours by using a normal size battery.
A conventional method for maintaining storage contents of a DRAM (memory backup) is to backup the entire DRAM, regardless of any condition, when the main power supply stops supplying power.
Generally, the battery used for backing up a DRAM is a secondary battery (rechargeable battery) since large current is necessary to back up the DRAM. Therefore, if the secondary battery discharges under any condition whenever the main power supply is turned off, as in the conventional case, there is no guarantee that the secondary battery will be charged enough when the main power supply is turned on. Accordingly, the secondary battery may not be charged enough in the event of a sudden electricity shut off, for which the secondary battery is originally provided in the facsimile apparatus.